1. Field of the Invention
The present invention relates to a power impact tool such as an impact driver or an impact wrench used for fastening a fastening member such as a bolt or a nut.
2. Description of the Related Art
In a power impact tool used for fastening a fastening member such as a bolt or a nut, it is preferable that a fastening operation is automatically completed by stopping the driving of a driving source such as a motor, when a torque for fastening the fastening member reaches to a predetermined reference value previously set.
In a first conventional power impact tool shown in publication gazette of Japanese Patent Application 6-91551, an actual torque, which is necessary for fastening the fastening member, is sensed and the driving of a motor is stopped when the actual torque reaches to a predetermined reference value. The first conventional power impact tool which stops the driving of the motor corresponding to the actual torque for fastening the fastening member needs a sensor provided on an output shaft for sensing the actual torque, so that it causes the cost increase and the damage of the usability owing to the upsizing of the power impact tool, even though the automatic stopping of the driving of the motor can be controlled precisely corresponding to the actual torque.
In a second conventional power impact tool, for example, shown in publication gazette of Japanese Patent Application 4-322974, a number of impact of a hammer is sensed and driving of a motor is automatically stopped when the number of impact reaches to a predetermined reference number, which is previously set or calculated from a torque inclination after the fastening member is completely fastened. The second conventional power impact tool, however, has a disadvantage that a large difference may occur between a desired torque and the actual torque for fastening the fastening member, even though the control for stopping the motor can easily be carried out. The difference causes loosening of the fastening member due to insufficient torque when the actual torque is much smaller than the desired torque. Alternatively, the difference causes to damage the component to be fastened by the fastening member or to damage a head of the fastening member due to superfluous torque when the actual torque is much larger than the desired torque.
In a third conventional power impact tool shown in publication gazette of Japanese Patent Application 9-285974, a rotation angle of a fastening member per each impact is sensed and driving of a motor is stopped when the rotation angle becomes less than a predetermined reference angle. Since the rotation angle of the fastening member per each impact is inversely proportional to the torque for fastening the fastening member, it controls the fastening operation corresponding to the torque for fastening the fastening member, in theory. The power impact tool using a battery as a power source, however, has a disadvantage that the torque for fastening the fastening member largely varies due to the drop of voltage of the battery. Furthermore, the torque for fastening the fastening member is largely affected by the hardening of a material of a component to be fastened by the fastening member.
For solving the above-mentioned problems, in a fourth conventional power impact tool shown in publication gazette of Japanese Patent Application 2000-354976, an impact energy and a rotation angle of the fastening member per each impact are sensed, and the driving of the motor is stopped when a torque for fastening the fastening member calculated with using the energy and the rotation angle becomes equal to or larger than a predetermined reference value. The impact energy is calculated with using a rotation speed of the output shaft at the moment when the output shaft is impacted, or a rotation speed of a driving shaft of the motor just after the impact. Since the fourth conventional power impact tool senses the impact energy based on an instantaneous speed at the impact occurs, it needs a high-resolution sensor and a high-speed processor, which is the cause of expensiveness.